Hydrogen sensor for fuel processors of a fuel cell

ABSTRACT

A method and apparatus estimate hydrogen concentration in a reformate stream produced by a fuel processor of a fuel cell. A sensor measures carbon monoxide, carbon dioxide, and water in the reformate stream. A fuel meter controls fuel input to the fuel processor. An air meter controls air input to the fuel processor. A water meter controls water input to the fuel processor. A transport delay estimator recursively estimates transport delay of the fuel processor. A hydrogen estimator associated with the transport delay estimator, the air, water and fuel meters, and the sensor estimates hydrogen concentration in the reformate stream. The hydrogen estimator includes a fuel processor model that is adjusted using the estimated transport delay. The carbon monoxide, the carbon dioxide and the water are measured using a nondispersive infrared (NDIR) sensor.

FIELD OF THE INVENTION

[0001] The present invention relates to fuel cells, and moreparticularly to a hydrogen sensor for a fuel processor of a fuel cell.

BACKGROUND OF THE INVENTION

[0002] Fuel cells are increasingly being used as a power source in awide variety of different applications. Fuel cells have also beenproposed for use in electrical vehicle power plants to replace internalcombustion engines. A solid-polymer-electrolyte membrane (PEM) fuel cellincludes a membrane that is sandwiched between an anode and a cathode.To produce electricity through an electrochemical reaction, hydrogen(H₂) is supplied to the anode and air or oxygen (O₂) is supplied to thecathode.

[0003] In a first half-cell reaction, dissociation of the hydrogen (H₂)at the anode generates hydrogen protons (H⁺) and electrons (e⁻). Themembrane is proton conductive and dielectric. As a result, the protonsare transported through the membrane while the electrons flow through anelectrical load that is connected across the electrodes. In a secondhalf-cell reaction, oxygen (O₂) at the cathode reacts with protons (H⁺),and electrons (e⁻) are taken up to form water (H₂O).

[0004] The main function of a fuel processor in the fuel cell system isto provide a continuous stream of hydrogen to the fuel cell stack thatconverts the chemical energy in the hydrogen fuel to electrical power.The fuel processor produces a reformate stream that is composedprimarily of hydrogen, carbon dioxide, nitrogen, water, methane andtrace amounts of carbon monoxide. During operation, the fuel cell stackdemands a certain flow rate of hydrogen from the fuel processor to meetthe vehicle's demand for power. The performance of the fuel processor ischaracterized by the flow rate of hydrogen in the reformate stream. Thecontrol of the fuel processor to maintain or track a desired flow rateof hydrogen by the fuel cell stack requires a feedback signal thatmeasures the hydrogen flow rate. The feedback signal is used in acontrol algorithm to take corrective action.

[0005] Currently, there is no hydrogen sensor technology that canmeasure the hydrogen concentration in the reformate stream that issuitable for use in fuel cell applications. For example, existinghydrogen sensors, such as those formed from a thin film of palladium,cannot be used in the presence of water droplets that are present in thereformate stream. Therefore, the thin film palladium sensors require awater filter that slows the response time. Furthermore, oxide-basedsensors such as ZrO₂ and SnO₂ need to operate in an oxidizingenvironment. The reformate, on the other hand, is a reducing environmentthat lacks oxygen. Thermal conductance sensors have also been proposed.However, these sensors cannot withstand the flowrates that areencountered in a fuel cell (typically 20 g/s of flow). Proton exchangemembrane (PEM) and metal hydride sensors have also been proposed butnone are currently commercially available.

SUMMARY OF THE INVENTION

[0006] A hydrogen sensor according to the present invention estimatesthe hydrogen concentration in a reformate stream produced by a fuelprocessor of a fuel cell. A sensor measures the concentration of carbonmonoxide, carbon dioxide, and water vapor in the reformate stream. Afuel meter measures and controls fuel input to the fuel processor. Anair meter measures and controls air input to the fuel processor. A watermeter measures and controls water input to the fuel processor. Atransport delay estimator connected to the sensor recursively estimatestransport delay of the fuel processor. A hydrogen estimator connected tothe transport delay estimator, the air, water and fuel meters, and thesensor estimates the hydrogen concentration in the reformate stream.

[0007] In other features of the invention, the hydrogen estimatorincludes a fuel processor model that is adjusted using the estimatedtransport delay. The concentration of carbon monoxide, carbon dioxideand water in the reformate stream are preferably sensed using anondispersive infrared (NDIR) sensor.

[0008] Further areas of applicability of the present invention willbecome apparent from the detailed description provided hereinafter. Itshould be understood that the detailed description and specificexamples, while indicating the preferred embodiment of the invention,are intended for purposes of illustration only and are not intended tolimit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

[0010]FIG. 1 illustrates a cross-section of a membrane electrodeassembly (MEA) of a fuel cell assembly;

[0011]FIG. 2 is a schematic block diagram of a control system for a fuelcell;

[0012]FIG. 3 is a schematic block diagram illustrating a hydrogen sensorfor a fuel cell; and

[0013]FIG. 4 illustrates steps for determining hydrogen concentrationusing the hydrogen sensor of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The following description of the preferred embodiment(s) ismerely exemplary in nature and is in no way intended to limit theinvention, its application, or uses.

[0015] Referring now to FIG. 1, a cross-section of a fuel cell assembly10 that includes a membrane electrode assembly (MEA) 12 is shown.Preferably, the MEA 12 is a proton exchange membrane (PEM). The MEA 12includes a membrane 14, a cathode 16, and an anode 18. The membrane 14is sandwiched between the cathode 16 and the anode 18.

[0016] A cathode diffusion medium 20 is layered adjacent to the cathode16 opposite the membrane 14. An anode diffusion medium 24 is layeredadjacent to the anode 18 opposite the membrane 14. The fuel cellassembly 10 further includes a cathode flow channel 26 and anode flowchannel 28. The cathode flow channel 26 receives and directs oxygen orair (O₂) from a source to the cathode diffusion medium 20. The anodeflow channel 28 receives and directs hydrogen (H₂) from a source to theanode diffusion medium 24.

[0017] In the fuel cell assembly 10, the membrane 14 is a cationpermeable, proton conductive membrane having H⁺ ions as the mobile ion.The fuel gas is hydrogen (H₂) and the oxidant is oxygen or air (O₂). Theoverall cell reaction is the oxidation of hydrogen to water and therespective reactions at the anode 18 and the cathode 16 are as follows:

H₂=2H⁺+2e⁻

0.5 O₂+2H⁺+2e⁻=H₂O

[0018] Since hydrogen is used as the fuel gas, the product of theoverall cell reaction is water. Typically, the water that is produced isrejected at the cathode 16, which is a porous electrode including anelectrocatalyst layer on the oxygen side. The water may be collected asit is formed and carried away from the MEA 12 of the fuel cell assembly10 in any conventional manner.

[0019] The cell reaction produces a proton exchange in a direction fromthe anode diffusion medium 24 towards the cathode diffusion medium 20.In this manner, the fuel cell assembly 10 produces electricity. Anelectrical load 30 is electrically connected across a first plate 32 anda second plate 34 of the MEA 12 to receive the electricity. The plates32 and/or 34 are bipolar plates if a fuel cell is adjacent to therespective plate 32 or 34 or end plates if a fuel cell is not adjacentthereto.

[0020] Referring now to FIG. 2, a control system 50 for a fuel cellstack 54 is illustrated. A fuel processor 56 generates a reformatestream 58 that is input to the anode flow channel 28 of the fuel cellstack 54. An air metering device 60 varies the input of air to the fuelprocessor 56. A fuel metering device 64 varies the input of fuel such asmethanol to the fuel processor 56. A water metering device 68 varies theinput of water to the fuel processor 56.

[0021] A hydrogen sensor 70 senses the hydrogen concentration of thereformate stream 58 and provides a hydrogen feedback signal 74 to a fuelprocessor controller 76. The fuel processor controller 76 sends controlsignals to the air metering device 60, the fuel metering device 64, andthe water metering device 68. A stack controller 80 provides a hydrogensetpoint signal 82 to the fuel processor controller 76. A currentsetpoint 86 is input to the stack controller 80. A current sensor 90senses the current that is output by the fuel cell stack 54 and providesa current feedback signal 92 to the stack controller 80.

[0022] In use, the air, the fuel and the water are supplied to the fuelprocessor 56. The fuel processor 56 produces the reformate stream 58.The hydrogen sensor 70 senses the hydrogen concentration of thereformate stream 58 and provides the hydrogen feedback signal 74 to thefuel processor controller 76. The fuel cell stack 54 produceselectricity from the hydrogen in the reformate stream 58. The currentsensor 90 senses the current output by the fuel cell stack 54 andgenerates the current feedback signal 92 that is input to the stackcontroller 80. The stack controller 80 compares the current feedbacksignal 92 to the current setpoint signal 86. The stack controller 80generates the hydrogen setpoint signal 82 that is output to the fuelprocessor controller 76.

[0023] Commercially available hydrogen sensors 70 have been found to beunsuitable for use in fuel cells used in electric vehicles. FIG. 3illustrates a hydrogen sensor 120 according to the present inventionthat indirectly measures hydrogen concentration from other components ofthe reformate stream. A carbon monoxide sensor 124 senses theconcentration of carbon monoxide in the reformate stream 58. A carbondioxide sensor 128 senses the concentration of carbon dioxide in thereformate stream 58. A water sensor 132 senses the concentration ofwater in the reformate stream 58. In preferred embodiment, the sensors124, 128 and 132 are a nondispersive infrared (NDIR) sensor 136available from Ion Optics, Inc. The NDIR sensor 136 detects theconcentrations of carbon monoxide, carbon dioxide and water influctuating pressure, temperature and flowrate conditions.

[0024] Outputs from the sensors 124, 128 and 132 are input to a fuelprocessor model 140 that forms part of the hydrogen sensor 120. The fuelprocessor model 140 models the operation of the fuel processor 56 aswill be described more fully below and provides a hydrogen feedbacksignal 142 to the fuel processor controller 76. Outputs from the airmetering device 60, the fuel metering device 64, and the water meteringdevice 68 are input to the fuel processor model 140. Outputs from thesensors 124, 128 and 132 are also input to a time delay estimator 144.The time delay estimator 144 determines the time required for materialsto pass through the fuel processor 56.

[0025] A basic feature of the fuel processor model 140 is to representthe fuel processor 56 as a black box in which the following overallchemical reaction occurs:

aC₈H₁₈+bO₂+cH₂O+kN₂→dH₂+eCO+fCO₂+gH₂0+hCH₄+kN₂

[0026] The coefficients such as a, b, and c, are the molar flow ratesinto the fuel processor 56 at any given time. Coefficients a, b, and care estimated from the air, fuel and water metering devices 60, 64 and68 that are input to the fuel processor 56. The sensors 124, 128 and 132or the combined NDIR sensor 136 provide the concentration of carbonmonoxide, carbon dioxide, and water in the reformate stream 58. The fuelprocessor model 140 and the transport delay estimator 144 assumeelemental balancing of carbon, hydrogen and oxygen. The fuel processormodel 140 and the transport delay estimator 144 also assume overall massbalancing to estimate the transport delay in fuel processor 56 using thetransport delay estimator 144. If not, then some of the model parametersare adjusted or optimized to drive the transport delay to a small value.

[0027] The transport delay is defined as the time that the elementsremain in fuel processor 56. The transport delay is a lumped parameterthat is adjusted to satisfy the elemental and mass balances. Theresulting information is used to estimate the concentration of hydrogen.The fuel processor model 140 may also be used to predict methaneconcentration in the reformate stream 58.

[0028] There are several underlying assumptions that are made by thefuel processor model 140 in modeling the fuel processor 56. The fuelprocessor model 140 assumes that there is no hydrocarbon slip exceptmethane and that there is no oxygen slip. The fuel processor model 140assumes that the fuel processor 56 is a train of plug flow reactors. Thefuel processor model 140 assumes that there is mass balance. In otherwords, the fuel processor 56 cannot create or destroy mass. The fuelprocessor model 140 assumes elemental balance. In other words, the fuelprocessor 56 cannot create or destroy elements. Finally, the fuelprocessor model 140 assumes that the fuel processor 56 has a variablelag.

[0029] Referring now to FIG. 4, steps for estimating hydrogenconcentration in the reformate stream 58 are illustrated. In step 200,carbon monoxide, carbon dioxide and water are measured in the reformatestream 58. In step 202, flowmeters measure the fuel, the air and thewater supplied to the fuel processor 56. In step 204, the transportdelay is recursively estimated to best satisfy elemental and massbalancing. In step 208, the fuel processor model 140 is corrected andthe hydrogen concentration is estimated.

[0030] Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, the specification and the following claims.

What is claimed is:
 1. A fuel cell system, comprising: a fuel cellstack; a fuel cell processor that provides a reformate stream to saidfuel cell stack; a carbon monoxide sensor that senses the concentrationof carbon monoxide in said reformate stream; a carbon dioxide sensorthat senses the concentration of carbon dioxide in said reformatestream; a water sensor that senses the concentration of water in saidreformate stream; a fuel meter for controlling fuel to said fuel cellprocessor; a water meter for controlling water to said fuel cellprocessor; an air meter for controlling air to said fuel cell processor;and a hydrogen sensor connected to said carbon monoxide sensor, saidcarbon dioxide sensor, said water sensor, said fuel meter, said watermeter, and said air meter, wherein said hydrogen sensor estimateshydrogen concentration of said reformate stream.
 2. The fuel cell systemof claim 1 wherein said hydrogen sensor includes a fuel processor modeland a transport delay estimator.
 3. The fuel cell system of claim 2wherein said transport delay estimator calculates transport delay byassuming elemental balance.
 4. The fuel cell system of claim 2 whereinsaid transport delay estimator calculates transport delay by assumingmass balance.
 5. The fuel cell system of claim 2 wherein said transportdelay estimator calculates said transport delay by balancing elementsincluding carbon, hydrogen and oxygen.
 6. The fuel cell system of claim2 wherein said hydrogen sensor calculates said hydrogen concentration byassuming elemental balance.
 7. The fuel cell system of claim 3 whereinsaid hydrogen sensor calculates said hydrogen concentration by assumingmass balance.
 8. A method of estimating hydrogen concentration of areformate stream produced by a fuel processor of a fuel cell, comprisingthe steps of: measuring carbon monoxide, carbon dioxide, and water insaid reformate stream produced by said fuel processor; measuring fuel,air and water that is input to said fuel processor; estimating hydrogenconcentration using said air, fuel and water inputs and said carbonmonoxide, said carbon dioxide, and said water in said reformate stream.9. The method of claim 8 further comprising the steps of: estimating atransport delay of said fuel processor; and adjusting a fuel processormodel using said estimated transport delay.
 10. The method of claim 9wherein said transport delay is estimated recursively.
 11. The method ofclaim 8 wherein said carbon monoxide, said carbon dioxide and said waterare measured using a nondispersive infrared (NDIR) sensor.
 12. Themethod of claim 8 wherein said step of estimating hydrogen concentrationassumes elemental balance.
 13. The method of claim 8 wherein said stepof estimating hydrogen concentration assumes mass balance.
 14. A systemfor estimating hydrogen concentration in a reformate stream produced bya fuel processor of a fuel cell, comprising: a sensor for measuringcarbon monoxide, carbon dioxide, and water in said reformate stream; afuel meter that meters fuel input to said fuel processor; an air meterthat meters air input to said fuel processor; a water meter that meterswater input to said fuel processor; and a hydrogen estimator connectedto said air meter, water meter said fuel meter, and said sensor thatestimates hydrogen concentration in said reformate stream.
 15. Thesystem of claim 14 wherein said hydrogen estimator estimates transportdelay and includes a fuel processor model that is adjusted using saidestimated transport delay.
 16. The system of claim 15 wherein saidtransport delay is estimated recursively.
 17. The system of claim 14wherein said carbon monoxide, said carbon dioxide and said water aremeasured using a nondispersive infrared (NDIR) sensor.
 18. The system ofclaim 15 wherein said fuel processor model of said hydrogen estimatorassumes elemental balance.
 19. The system of claim 15 wherein said fuelprocessor model of said hydrogen estimator assumes mass balance.